The present invention relates to aqueous coating compositions used, for example, as water-based paints, and which crosslink at room temperature.
Coating compositions which crosslink under ambient conditions are known in the prior art. The best known compositions are based on reactions between epoxides and carboxylic acids, between isocyanates and water, between polyaziridine and carboxylic acids, and between activated methylene and acrylic acid.
Compositions involving the reaction between an epoxide and a carboxylic acid comprise a binder which is either the mixture of a latex containing a carboxylic acid function and a latex containing an epoxide function, or a latex containing both of these functions at the same time, such as, for example, the one described in EP 653,469.
It has been observed that these binders are highly unstable on storage: the crosslinking reactions between the two acid and epoxide functions take place during this storage, to the extent that the reactive function of the binder does not take place during use of the binder as a coating. This creates a film with poor properties, in particular one with poor chemical resistance.
One aim of the present invention is thus to propose coating compositions based on a mixture of latex containing carboxylic acid functions and epoxy functions, which behave satisfactorily on storage and have good reactivity at the time of use.
With these aims, the invention relates to a binder for a coating composition, comprising:
at least one latex (A) prepared from ethylenically unsaturated monomers and containing a carboxylic acid function, and
at least one latex (B) prepared from ethylenically unsaturated monomers and containing an epoxide function, and for which the particle size of the said latices is not more than 50 nm.
The invention also relates to a coating composition based on a binder as defined above.
Other details and advantages of the invention will emerge even more clearly on reading the description and the examples.
The invention thus relates, firstly, to a binder for a coating composition, comprising:
at least one latex (A) prepared from ethylenically unsaturated monomers and containing a carboxylic acid function, and
at least one latex (B) prepared from ethylenically unsaturated monomers and containing an epoxide function, and for which the particle size of the said latices is not more than 50 nm.
The term latex is understood to refer to an aqueous dispersion of polymer particles.
According to the invention, the binder is a mixture of latices prepared from ethylenically unsaturated monomers. This mixture comprises at least one latex containing a carboxylic acid function and at least one latex containing an epoxide function. The essential characteristic of the binder according to the invention is that the particles of these latices contain carboxylic acid and epoxide functions and are not more than 50 nm in size, preferably between 20 and 40 nm in size. The term size is understood to refer to the particle diameter. The particle size distribution of the particles in each latex is relatively unimportant. On the other hand, it is essential for all of the particles in the latex to be not more than 50 nm in size: the coarsest latex particles are thus not more than 50 nm in size. The particle size is measured by transmission electron microscopy (TEM).
Preferably, the molecular mass {overscore (M)}n of the polymer in the latex (A), which contains a carboxylic acid function, is between 40,000 and 500,000 g/mol, preferably between 100,000 and 400,000 g/mol.
For the latex (B), which contains an epoxide function, the molecular mass {overscore (M)}n is preferably between 4000 and 50,000 g/mol, even more preferably between 20,000 and 40,000 g/mol.
These molecular masses are measured by gel chromatography (GPC).
In general, the minimum film-forming temperature (MFFT) of the polymer in the latex (A), which contains a carboxylic acid function, is between 0 and 15xc2x0 C., preferably between 6 and 12xc2x0 C., and that of the polymer in the latex (B), which contains an epoxide function, is between 30 and 50xc2x0 C., preferably between 35 and 45xc2x0 C.
In general, the latex (A), which contains a carboxylic acid function, and the latex (B), which contains an epoxide function, are present in the binder in an xe2x80x9cequimolarxe2x80x9d ratio, i.e. the latices are introduced in amounts such that the mixture contains one carboxylic acid function per epoxide function.
It has been observed that the best results are obtained when the epoxide groups in the latex (B) are at the surface of the particles in the said latex. It is thus preferable to use a latex (B) in which most of the epoxide functions are at the surface of the polymer particles.
The binder according to the invention is obtained by simple mixing of the latices (A) and (B).
The latices (A) and (B) are generally derived from the polymerization of ethylenically unsaturated monomers such as:
(meth)acrylic esters in which the alkyl group contains from 1 to 20 carbon atoms, such as methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, isopropyl acrylate, butyl acrylate, butyl methacrylate, 2-ethylhexyl methacrylate, decyl acrylate, lauryl methacrylate or benzyl acrylate,
vinylaromatics such as styrene, vinyltoluene or xcex1-methylstyrene.
The latex (A) is derived from the polymerization of at least one of the said ethylenically unsaturated monomers and from at least one monomer carrying the carboxylic acid function. In general, the carboxylic acid function originates from a mono- or polycarboxylic acid containing one polymerizable bond per molecule. In general, such acids contain 3 to 24 carbon atoms and one or two carboxylic acid groups per molecule. They may be, for example:
acrylic, methacrylic, maleic, fumaric, itaconic, ethacrylic, crotonic or citraconic acid,
semi-esters of dicarboxylic acids in which the esterified alcohol group comprises 1 to 20 carbon atoms, such as: methyl hydrogen fumarate, benzyl hydrogen maleate, octyl hydrogen itaconate or dodecyl hydrogen citraconate, and the like.
The preferred carboxylic acids are acrylic acid and methacrylic acid.
The monomers carrying the carboxylic acid function generally represent at least 15% by weight of the sum of the monomers forming the polymer in the latex (A).
The latex (B) is derived from the polymerization of at least one of the above ethylenically unsaturated monomers and of at least one monomer carrying the epoxide function. Preferably, the epoxide function originates from a glycidyl (meth)acrylate monomer. Such a monomer can be obtained commercially. It can be derived from the esterification reaction of acrylic acid or methacrylic acid with glycidol or epichlorohydrin, followed by a adehydrohalogenation.
The monomers carrying the epoxide function generally represent 10 to 40% by weight of the sum of the monomers forming the polymer in the latex (B), preferably 20 to 30%.
These latices (A) and (B), which have a particle size of not more than 50 nm, can be obtained by the standard polymerization processes such as, for example, the one described in patent application EP 644,205, and in particular Example 4 of that application. This involves polymerization of monomers emulsified in water. Preferably, these processes are carried out continuously. According to this Example 4, the latex is obtained by preparing an aqueous solution comprising the surfactant and the polymerization initiator, into which the monomer mixture is introduced portionwise and continuously.
The surfactants used to emulsify the monomers are especially anionic surfactants.
As examples of anionic surfactants which may be used, mention may be made of linear sodium and potassium alkyl sulphates such as sodium lauryl sulphate, sodium alkylaryl sulphates, ethoxylated alkylphenol sulphates and sulphonates, ethoxylated fatty alcohol sulphates and sulphonates, and sulphosuccinates.
The molecular masses of the latices (A) and (B) are controlled by the amount of transfer agent introduced during the polymerization.
The amounts of surfactants used in the emulsion polymerization process range between 0.01 and 10% by weight in general, and preferably between 0.2 and 5% by weight, relative to the total weight of the monomers and of water.
It is possible to orient the polymerization of the latex (B) such that most of the epoxide functions are at the surface of the particles by carrying out a continuous polymerization processxe2x80x94rather than a batchwise processxe2x80x94according to the process defined in patent application EP 644,205, and in particular Example 4 of that application.
One characteristic of the binder according to the invention is that it leads to coatings with only one glass transition temperature (Tg). This proves that there has been good interaction between the acid and epoxide polymer chains and that the coating obtained is homogeneous.
The invention also relates to a coating composition containing common adjuvants and the binder according to the invention.
In addition to the binder according to the invention, the composition can contain any binder used in coating compositions of this type. Mention may be made of acrylate and styrene/acrylate latices, acetate copolymers, aqueous polyurethane, polyester, epoxy and alkyd dispersions, and polyurethane, epoxy and alkyd resins. These binders can replace up to 80% by weight, preferably up to 50% by weight, of the binder according to the invention in the aqueous coating compositions.
When they are intended for use as binders in paints, the aqueous coating compositions according to the present invention can be applied to a large number of substrates to form films and coatings which crosslink and give compositions that thermoset at room temperature or above. The preferred crosslinking temperatures range between 10 and 100xc2x0 C.
The compositions can contain as common adjuvants: thickeners, antifoaming agents, glycols, coalescence agents, pigments, base activators. These adjuvants usually represent less than 5% by weight of solids present in the final aqueous composition.
The compositions can be applied to any type of substrate: wood, metal, concrete, plastic, glass and the like.